Structural adhesives can be defined as materials used to bond other high strength materials, such as wood, composites, or metal, so that the practical adhesive bond strength is in excess of 6.9 MPa (1,000 psi) at room temperature. Structural adhesives are used in a variety of applications that include general-use industrial applications, as well as high-performance applications in the automotive and aerospace industries. Structural adhesives may be used to replace or augment conventional joining techniques such as screws, bolts, nails, staples, rivets and metal fusion processes (e.g., welding, brazing and soldering). In particular, in the transportation industry (e.g., automotive, aircraft or watercraft), structural adhesives can present a light weight alternative to mechanical fasteners. To be suitable as structural adhesives, the adhesives should have high mechanical strength and impact resistance.
The inherent brittleness of heat-cured epoxy-based adhesives can be overcome by adding toughening agents to the adhesive compositions which impart greater impact resistance to the cured epoxy compositions. Typically, a rather large amount of toughening agent may have to be employed to achieve satisfying toughening and/or impact resistance. However, large amounts of toughening agents can lead to an increased viscosity of the adhesive composition and poor handling. Therefore, there is a need for providing compositions, in particular compositions suitable as structural adhesives, having the same or even improved toughening effect and/or impact resistance at a lower level of toughening agent.